It's possible to use the iPad as a USB host and connect up class-compliant USB audio interfaces through the Camera Connection Kit.
But is there a way to adjust their settings in iOS? There is in OS X through the Audio/MIDI Setup:
The device's output level, for instance, is affected by the regular iPad volume controls, but the playthrough feature and input gain don't seem to have any controls, at least not in Garageband. Is there a hidden setting somewhere or an app that can control such things?